Implementing multi user multiple input multiple output (MU MIMO) base station using single-user (SU) MIMO co-located base stations

ABSTRACT

A system that implements multi user multiple inputs multiple outputs (MU MIMO) base station using a plurality of co-located single-user (SU) MIMO base stations is provided herein. The system may include a number N co-located single-user multiple input multiple output (SU-MIMO) bases stations each having a number K MIMO rank, wherein said N co-located SU-MIMO base stations are configured to share a common antennas array, operating over a common frequency band; a front-end MIMO processor connected to said N co-located SU-MIMO base stations and further coupleable to said common antennas array; and a back-end coordinator configured to collaboratively assist in optimizing operation of said N co-located SU-MIMO base stations, such that said N co-located SU-MIMO base stations and said front-end MIMO processor collaboratively implement a multi-user multiple input multiple output (MU-MIMO) base station capable of dynamically separating a coverage area into N*K spatial channels.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/172,500, filed on Feb. 4, 2014, which is acontinuation-in-part application of U.S. patent application Ser. No.13/888,057, filed on May 6, 2013, which claims benefit of U.S.Provisional Patent Application Nos. 61/762,486, filed on Feb. 8, 2013and 61/811,751, filed on Apr. 14, 2013; U.S. patent application Ser. No.14/172,500 further claims benefit of U.S. Provisional Patent ApplicationNos. 61/845,270, filed on Jul. 11, 2013 and 61/898,817, filed on Nov. 1,2013, all of which are incorporated in their entirety herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to multi user multiple inputmultiple outputs (MU MIMO) base stations, and more specifically, suchbase stations which are implementing MU MIMO using time division duplex(TDD).

BACKGROUND OF THE INVENTION

Prior to setting forth a short discussion of the related art, it may behelpful to set forth definitions of certain terms that will be usedhereinafter.

The term “Small Cell” as used herein is defined as a low-powered radioaccess node, or base station that operates in licensed spectrum thathave a range of 10 meters to 1 or 2 kilometers, compared to a mobilemacrocell which might have a range of a few tens of kilometers. Smallcells are a vital element to 3G/4G data off-loading, and many mobilenetwork operators see small cells as vital to managing Long TermEvolution (LTE) advanced spectrum more efficiently compared to usingjust macrocells. The primary use of small cells is to increase capacityof the traffic for the operators rather than increase the mere coverageof the network.

The term “multiple-input-multiple-output” or “MIMO” as used herein, isdefined as the use of multiple antennas at both the transmitter andreceiver. MIMO systems may improve communication performance by offeringsignificant increases in data throughput and link range withoutadditional bandwidth or increased transmit power. MIMO systems mayachieve this goal by spreading the transmit power over multiple antennasto achieve spatial multiplexing that improves the spectral efficiency(more bits per second per frequency range or hertz (Hz) of bandwidth) orto achieve a diversity gain that improves the link reliability (e.g.reduced fading), or increased antenna directivity.

The term “multi-user multiple-input-multiple-output” or “MU-MIMO” asused herein is defined as a wireless communication system in whichavailable antennas are spread over a multitude of independent accesspoints and independent radio terminals—each having one or multipleantennas. In contrast, single-user MIMO considers a single multi-antennatransmitter communicating with a single multi-antenna receiver. Toenhance the communication capabilities of all terminals, MU-MIMO appliesan extended version of space-division multiple access (SDMA) to allowmultiple transmitters to send separate signals and multiple receivers toreceive separate signals simultaneously in the same frequency band.

The term “Time-division duplexing” or “TDD” as used herein is defined asis the application of time-division multiplexing to separate outward andreturn signals. It emulates full duplex communication over a half-duplexcommunication link. Time-division duplexing has a strong advantage inthe case where there is asymmetry of the uplink and downlink data rates.As the amount of uplink data increases, more communication capacity canbe dynamically allocated, and as the traffic load becomes lighter,capacity can be taken away. The same applies in the downlink direction.For radio systems that aren't moving quickly, hereinafter referred to as“quasi-static” stations, another advantage is that the uplink anddownlink radio paths are likely to be very similar. This means thattechniques such as beamforming work well with TDD systems.

In some MU-MIMO that are already known in the art, a spatial separationmechanism creates multiple separated channels between the base stationand the users or the same spectrum; sub sets of users population areassigned to these different spatial channels; a common basestationscheduler makes sure that simultaneous users that may experienceMU-MIMO's self-inflicted cross talk, are served over non overlappingPRBs, thus maintaining efficient spectrum multiplexing.

According to prior art MU-MIMO systems, the aggregated data rate of anN-user MIMO is slightly below N times the data rate of aSingle-User-MIMO (SU-MIMO), due to random distribution of the users,channel estimation errors, mobility, and the projected signal levelsdependence on pairing of users, which impacts MCS estimation accuracy.Such MU-MIMO base stations serve as N unified legacy base stations,sharing common channel estimation and common MIMO processing blocks, aswell as common Radio Resources Control (RRC) and a common scheduler.

FIG. 1A is a block diagram of a system 100 and illustrating the functionstacks of single-user MIMO system according to the prior art. FIG. 1B isa block diagram illustrating the function stacks of a multi-user MIMOsystem, according to the prior art. For SU-MIMO, system 100 may include:higher media access control (MAC) 110 may perform scheduling for theMIMO operation; lower MAC 120 which may handle (for example, multiplex,de-multiplex, modulation, and demodulation) the multiple data streamsfor the MIMO; pre-coding function 130 which may transmit each of themultiple data streams through the multiple transmit antennas accordingthe pre-coding weight.

In order to upgrade the SU-MIMO system into a MU-MIMO system, one maymodify the scheduler in higher MAC 110 to a new higher MAC 160 of system150 to coordinate the multiple users. Additionally, one may multiply thedata handling function in lower MAC 120 to newer lower MAC 170, ofsystem 150, to accommodate more users simultaneously. Furthermore, onemay modify the pre-coding function 130 to the newer one 180 as in system150 so that each data stream from all MU-MIMO users may transmit throughall the transmit antennas simultaneously.

SUMMARY OF THE INVENTION

The present invention, in embodiments thereof, may provide a system andmethod which may implement multi user multiple inputs multiple outputs(MU MIMO) base station using single-user (SU) MIMO co-located basestations. Embodiments of the system of the present invention may includea number N co-located single-user multiple input multiple output(SU-MIMO) bases stations each having a number K MIMO rank, wherein saidN co-located SU-MIMO base stations are configured to share a commonantennas array, operating over a common frequency band; a front-end MIMOprocessor connected to said N co-located SU-MIMO base stations andfurther coupleable to said common antennas array; and a back-endcoordinator configured to collaboratively assist in optimizing operationof said N co-located SU-MIMO base stations, such that said N co-locatedSU-MIMO base stations and said front-end MIMO processor collaborativelyimplement a multi-user multiple input multiple output (MU-MIMO) basestation capable of or configured to dynamically (i.e., over time)separating a coverage area (being a portion of space which is covered bythe communication service of a base station) into N*K spatial channels.The spatial separation is achieved by using the data from each SU-MIMObase station for making educated requests to them for a bettercoordination between them.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best bemore fully understood by reference to the following detailed descriptionwhen read with the accompanying drawings in which:

FIGS. 1A and 1B are SU-MIMO and MU-MIMO respectively according to theprior art;

FIG. 2 depicts the function and architecture of a MU-MIMO implementationusing a front end processor, a coordinator, and 4 SU-MIMO baseband unitsin accordance with some embodiments of the present invention;

FIG. 3 depicts another aspect a MU-MIMO implementation using a front endprocessor, and four SU-MIMO baseband units in accordance with someembodiments of the present invention;

FIG. 4 illustrates a TDD MU-MIMO base station that includes N co-locatedk ranked SU-MIMO basebands in accordance with some embodiments of thepresent invention;

FIG. 5 depicts the function of the front end processor and theassociated signal/data flows in accordance with some embodiments of thepresent invention;

FIG. 6 shows the simulated signal loss for the MU-MIMO system using azero forcing algorithm without MU-MIMO scheduling;

FIG. 7 is a high level flowchart illustrating an aspect in accordancewith some embodiments of the present invention;

FIG. 8 is a high level flowchart illustrating another aspect inaccordance with some embodiments of the present invention; and

FIG. 9 is a high level flowchart illustrating yet another aspect inaccordance with some embodiments of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the present inventionwill be described. For purposes of explanation, specific configurationsand details are set forth in order to provide a thorough understandingof the present invention. However, it will also be apparent to oneskilled in the art that the present invention may be practiced withoutthe specific details presented herein. Furthermore, well known featuresmay be omitted or simplified in order not to obscure the presentinvention.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing,” “computing,”“calculating,” “determining,” or the like, refer to the action and/orprocesses of a computer or computing system, or similar electroniccomputing device, that manipulates and/or transforms data represented asphysical, such as electronic, quantities within the computing system'sregisters and/or memories into other data similarly represented asphysical quantities within the computing system's memories, registers orother such information storage, transmission or display devices.

The term “existing” as used herein relates to devices that are currentlyavailable “off the shelf” and are not modified while implemented in asystem in accordance with embodiments of the present invention. Examplesare existing SU-MIMO base stations which include existing basebandmodules, existing schedulers, and existing radio circuits.

The term “collaborative” as used herein relates a manner in whichseveral devices operate while taking into account the actions of theother devices, possibly through a coordinator.

The term “on-site” as used herein relates to operation of a system postdeployment in the real-life environment, as opposed to a lab or afactory.

FIG. 2 shows that the functions modification needed for a MU-MIMO system200 implementing the MU MIMO using SU-MIMO systems, in accordance withembodiments of the present invention. The MU-MIMO system may be carriedby front-end processor 210 and coordinator 220 and leave the functionsof the SU-MIMO systems which are co-located in one block 230 with aminimal change.

The functions that may be performed by coordinator 220 are describedherein The functions of the bank of co-located SU-MIMO basebands (orbase stations) 230 are those of the SU-MIMO bases stations that arecurrently available. These functions of SU-MIMO may be reused forembodiments of the present invention of creating a MU-MIMO base stationwith a minimal modification. An interconnection device 240 (for example,a local area network) may provide the interconnection among front-endprocessor 210, coordinator 220 and bank of co-located SU-MIMO basebasebands (or base stations) 230. It should be noted that for thepurpose of the present application, co-located base stations are basestations modules that are electronically interconnected and usuallypacked on a same integrated circuit (IC) or at least same printedcircuit board (PCB). Through interconnection 240, each of the co-locatedbasebands may send its scheduling information to the coordinator 220 inadvance. Front-end processor 210 may send inseparable user equipment(UE) (not shown) identification (ID) (e.g., those UEs having thecrosstalk exceeding a set threshold) to coordinator; coordinator 220 maysend the MU-MIMO re-scheduling information to the engaged basebands (orbase stations); coordinator 220 may request the engaged basebands (orbase stations) to re-schedule their users' assigned resource blocks.

In contrast to a MU MIMO base station that necessitates uniqueapplication specific integrated circuit (ASIC), embodiments of thepresent invention illustrate how it would be possible to build a MU MIMObase station using only readily available components such as SU MIMOASICs and some logic components such as the front end processor and theback end coordinator. Such as system may include a number N co-locatedsingle-user multiple input multiple output (SU-MIMO) bases stations eachhaving a number K MIMO rank, wherein said N co-located SU-MIMO basestations are configured to share a common antennas array, operating overa common (i.e. same) frequency band; a front-end MIMO processorconnected to said N co-located SU-MIMO base stations and furthercoupleable to said common antennas array; and a back-end coordinatorconfigured to collaboratively assist in optimizing operation of said Nco-located SU-MIMO base stations, such that said N co-located SU-MIMObase stations and said front-end MIMO processor collaborativelyimplement a multi-user multiple input multiple output (MU-MIMO) basestation capable of or configured to dynamically separating a coveragearea into N*K spatial channels.

FIG. 3 shows the function stack of another MU-MIMO architecture in asystem 300 implementing the MU MIMO in accordance with embodiments ofthe present invention. The MU-MIMO system in accordance with embodimentsof the present invention may not need the coordination (i.e.,scheduling) among the users for the MU-MIMO operation. External MU-MIMOprocessor 310 may connect to each of the co-located SU-MIMO basebands(or base stations), 320. Unlike other external MU-MIMO architecture, Theexternal MU-MIMO architecture in accordance with embodiments of thepresent invention does not equip coordinator to perform scheduling forthe multiple users. The non-scheduling external MU-MIMO system may besubject to a performance loss, depending on the MU-MIMO algorithm used.

FIG. 4 illustrates system 400 implementing a TDD MU-MIMO base stationthat may include N co-located K ranked SU-MIMO basebands (with at leastone baseband on a base station). Each of the N basebands 440-1, 440-2 .. . 440-N may serve many users simultaneously with different resourcesblocks (frequency channels/time slots). The individual user may beserved by one baseband up to k-ranked MIMO (i.e., up to k data streams).These basebands share the M antennas 410-1, 410-2, . . . , 410-M and Mtransceiver radios 420-1, 420-2, . . . , 420-M) through a front-endprocessor (for example, a Field Programmable Gate Array—FPGA) 430. M maybe larger than or equal to N*K. The front end-end processor 430 mayprocess the digital beam-forming (e.g., pre-coding) to the transmitsignals and process the channel estimation on the received signal forall the users. Coordinator 450, interconnected with all the basebandsand the front-end processor 430, may serve scheduler functions of theMU-MIMO operation for the co-located N basebands. The coordinator may bepart of the front-end processor. The interconnection 470 may be servedby a local area network (LAN). Basebands may connect to the backhaul 460through interconnection 470.

According to some embodiments of the present invention, the overallsystem may be operable in a Time Division Duplex (TDD) configuration,wherein each one of said N co-located SU-MIMO base stations has anexisting scheduler and wherein said front-end MIMO processor isconfigured to carry out said dynamic separation based on the existingschedulers.

According to some embodiments of the present invention, each one of saidN co-located SU-MIMO base stations has an existing radio module, whereinsaid front-end processor is further configured to carry out on site,periodic self-calibration on transmit and receive chains of each one ofsaid radio modules or antennas, wherein the self-calibration is usablefor establishing channel reciprocity, so that uplink channel informationis mapped into downlink channel estimation in a time division duplex(TDD) system.

According to some embodiments of the present invention, each one of saidN co-located SU-MIMO base stations has an existing baseband moduleconfigured to carry out channel estimation of the channels served by therespective SU-MIMO base stations, and wherein said front-end processoris further configured to carry out channel estimation for all thechannels served by said N co-located SU-MIMO base stations.

According to some embodiments of the present invention, the front-endprocessor may be further configured to: derive identification for eachof active user equipment (UE) and map said identifications intorespective channel estimations.

According to some embodiments of the present invention, theidentification derivation may be based on decoding the UE ID in thefront end processor and the respective base station.

According to some embodiments of the present invention, each of said Nco-located SU-MIMO base stations has an existing baseband moduleconfigured to detect uplink Physical Resource Block (PRB), each PRBbeing mappable to a distinct reference of frequency/time bins containingorthogonal frequency-division multiplexing OFDM symbols at a giventransmit time interval (TTI), wherein said front-end processor isfurther configured to tag a channel estimation of a user equipment (UE)based on a bin received from said existing baseband modules. Userequipment (“UE” or “a UE”) may be a device such as a cellular telephone,wireless-capable computer or laptop computer, smartphone, or otherwireless or cellular capable device.

FIG. 5 depicts, in greater detail functionality of system 500 and thefunction of the front end processor 530 and the associated signal/dataflows, according to embodiments of the present invention. The systemincludes a bank of M transmit/receive antennas 510-1 . . . 510-M. Eachantenna may be connected to one of the M radios 520. Each radio mayinclude up/down converter, digital-to-analog (DAC) and analog-to-digital(ADC) converters. The system further includes front-end processor 530.Front-end processor may perform the self-calibration on all transmit andreceive paths through the radios and antennas 530-1. A TDD system mayuse the calibration information to apply the channel reciprocity. Forexample, the calibration information may help converting the uplinkchannel estimation to the downlink channel information. Front-endprocessor may retrieve the UEs' Identification (MAC address) and itsassigned channel (resources blocks) from the downlink baseband signals530-2. The UEs ID and their assigned channels are needed for performingMU-MIMO in the case that the information is not passed from baseband inadvance. Front-end processor may perform the channel estimation on theuplink (received) signal for all the active UEs 530-3. Front-endprocessor may convert, with the transmit/receive path calibration data,the uplink channel information (estimation) to the downlink channelinformation 530-4. With the downlink channel information, the saidfront-end processor may calculate the downlink crosstalk (or SINR loss)among the active UEs. The said front-end processor may check if pairs ofUEs are not suitable for MU-MIMO operation (e.g., their crosstalk (orsignal loss) is above a pre-set threshold). Front-end processor may use,with the downlink channel information, any MU-MIMO algorithm (forexample, zero-forcing (ZF), minimum mean square error (MMSE), or anyproprietary or other suitable algorithm) to calculate the pre-codingweights (amplitudes and/or phases) for all the data streams and users tominimize the crosstalk in the MU-MIMO operation 530-5. The front-endprocessor may apply the weights to the signals of the UEs engagedMU-MIMO operation accordingly. Front-end processor may phase lock thelocal oscillators of all the radios to keep the signal coherency, whichmay be needed for MIMO operation 530-6.

The bank of collocated baseband 540 is also shown with signal/dataconnection 580-1 . . . 580-N between the N basebands and the saidfront-end processor. Each baseband may send the downlink baseband signalto and receive the uplink (received) base band signal from the front-endprocessor for each active UE.

Signal connection 590-1 . . . 590-M is shown between the M radios andthe front-end processor. Both uplink (received) and downlink (transmit)signals in these connections may be set in I and Q quadrature format.

Coordinator 550 may be interconnected with the front end processor andeach baseband through, for example, a local area network 570. Thecoordinator may receive the scheduler information (e.g., UEs' ID andtheir assigned resource blocks) from each baseband 540 in advance andmay obtain the DL channel information of each active UE from thefront-end processor 530. With this information, the coordinator maycheck if there is conflict (for example, the crosstalk exceeding acertain set threshold) among the UEs scheduled for MU-MIMO operation.The said coordinator may resolve a schedule conflict by re-schedulingUEs' assigned resource blocks and inform the associated baseband toprepare the baseband signals for the UEs accordingly. The coordinatormay also resolve a schedule conflict by informing the associatedbaseband to re-schedule the UEs' resource blocks. The function of thesaid coordinator may be performed by the front-end processor.

According to some embodiments of the present invention, the front-endprocessor is configured to derive uplink channel information relating toat least some user equipment (UE), wherein said uplink channelestimation is converted to downlink channel information by applyingchannel reciprocity, and further based on calibration data.

According to some embodiments of the present invention, a communicationlink established between each base station's baseband and the front endprocessor, over which schedulers plans are transferred in advance, tothe front end processor, using UE ID or UE reference via bin at a givenTTI, where said scheduled UEs per TTI, per PRB, are processed by theMU-MIMO mechanism

According to some embodiments of the present invention, a front-endprocessor is configured calculate a downlink pre-coding weights for atleast some user equipment (UE), camped on the base station using saiduplink channel information, said calibration data, and a MU-MIMO spatialmultiplexing algorithm.

According to some embodiments of the present invention, a communicationlink between said base stations and the front end processor is used toinform it of a quality of service (QoS) and the signal to interferenceplus noise (SINR) of each of the scheduled UEs in both down and uplinks.

According to some embodiments of the present invention, the MU-MIMOprocessing may take into account: the quality of service (QoS)requirement and the signal to interference plus noise (SINR) levels ofthe UEs to be scheduled, and choose whether to use Zero Forcing orminimal mean square error (MMSE) algorithms for antennae weightscalculation.

According to some embodiments of the present invention, the MU-MIMO usesZero Forcing (ZF) whenever signal to interference plus noise SINR levelsof the user equipment (UEs) to be scheduled are approximately at genericSINR level, where generic means service that could have been providedonly to one UE, and the MU-MIMO processing uses minimal mean squareerror (MMSE) whenever Zero Forcing is not used.

According to some embodiments of the present invention, the MU-MIMOprocess checks quality of service (QoS) tagging of the scheduled UEs,and if none is set, an MMSE weight calculation is done based on aproportional fairness approach.

According to some embodiments of the present invention, the MU-MIMOprocess checks quality of service QoS of the schedules UEs, and if allare tagged for the QoS, a minimal mean square error (MMSE) calculationis performed and prospective SINRs are compared to those required by thevarious QoS, and if found sufficient, the weights calculation isactually used, and if not, one of the UE's service is degraded, so thatthe other QoS requirements are fulfilled, and if not a second UEsservice is degraded.

According to some embodiments of the present invention, wherein theMU-MIMO process checks QoS of the schedules UEs, and if some of the UEsare tagged for QoS and some other are not, then the QoS ones takepriority, and the non QoS tagged ones are served according to aproportional fairness approach.

According to some embodiments of the present invention, each one of saidN co-located SU-MIMO base stations has an existing baseband module(i.e., the SU-MIMO base band module which is unmodified), and whereinsaid a back-end coordinator is further configured to receive from saidfront-end-processor an information regarding poorly separated userequipment (UEs) being UEs that are separated below a predefined level,and notifies at least one of the existing baseband modules to modify aschedule plan.

According to some embodiments of the present invention, wherein each oneof said N co-located SU-MIMO base stations has an existing radio modulehaving a local oscillator, wherein said front-end processor is furtherconfigured to phase lock on all said local oscillators, in order toachieve phase coherency, sufficient for beamforming.

According to some embodiments of the present invention, the front-endprocessor is further configured to send user equipment (UE) uplinkchannel information and UE identification to said back-end coordinatorfor checking and resolving conflicts occurring whenever a crosstalklevel exceeds predefined levels.

According to some embodiments of the present invention, each of said Nco-located SU-MIMO base stations has an existing scheduler (the SU-MIMObase scheduler which is unmodified) and wherein said back-endcoordinator is further configured to identify poorly separated UEs andhandover one or more of them to other base station in the system, andrepeat this procedures for other poorly separated UEs, so the occurrenceof poorly separated UEs is minimized.

According to some embodiments of the present invention, an intervention(i.e., an action carried out by a processor or a coordination in orderto resolve a conflict between several devices) is achieved by allowingsome schedulers to proceed as planned, while others are requested torefrain from scheduling a given UE for certain PRBs, over a given up andcoming Transmission Time Interval (TTI).

According to some embodiments of the present invention, such handoverevents are limited to UEs whose history indicated static position, wherehistory is measure in tens of seconds, and where static is defined aschannel estimation variation that has approximately zero mean.

According to some embodiments of the present invention, wherein theantenna array comprises M antennas, serving N baseband units eachsupporting K grade MIMO, where M≧N*K, i.e. the baseband population issparse, thus increasing the MU-MIMO degrees of freedom and promotinghigher and better spatial separation.

FIG. 6 shows in a pair of simulation graphs 600 that the simulatedsignal loss for the non-scheduling MU-MIMO operation using the publiclywell-known algorithms, zero-forcing, for the case 610 of antenna numbersbeing equal to the supported users' data streams (e.g., base stationuses 8 antennas—4 pairs of linear polarized antennas to support 4 UEswith rank 2 MIMO) and the case 620 of antenna numbers being twice of thesupported data streams (e.g., base station uses 16 antennas—8 pairslinear polarized antennas to support 4 UEs with rank 2 MIMO). Thissystem simulation is based on the use of 80 UE, each UE 2×2 MIMO; Sectorof 90°; range of 150 m; NLOS Rayleigh 6 rays; UE quads selected atrandom; 3,000 drops (runs). The simulation result of 620 shows that 90%of the UEs (data streams) is subject to less than 6.8 dB signal loss, orhalf of the UEs is subject to less than 3.7 dB signal loss. Thesimulation results shows that the signal loss may be insignificant forcertain systems without MU-MIMO scheduler, compromised for simplicity. Amore complicated algorithm, like MMSE or improved proprietary algorithm,may reduce the signal loss further.

FIG. 7 700 describes the uplink part MU-MIMO procedure for said thefront-end processor in the invented MU-MIMO TDD base station. Step 710shows that the front-end processor may perform the calibration ontransmit and receive paths of all radios/antennas and store or updatethe calibration data. This processor may repeat the calibrationroutinely with a pre-set timer and/or based on the temperature changeand update the calibration data, shown on step 720. Step 730 shows thatthe processor may channel estimate on all the uplink signals (pilots)from each of the receive antennas and retrieve the UE's IDs. Thisinformation is then stored or updated with the new channel estimation(every sub-frame). With the calibration data, the processor may thenconvert the uplink channel information into the downlink channelinformation for the TDD system, shown on Step 740. In addition, step 750shows that the processor may send the uplink baseband signals to thecorresponding served baseband once the UE's IDs are retrieved.Furthermore, step 760 shows that the processor may calculate thepre-coding weights and crosstalk for all the possible UE pairs using achosen MU-MIMO algorithm (for example, zero-forcing, MMSE, or anyproprietary or other suitable algorithm) with the updated downlinkchannel information. Step 770 shows that the processor may inform thesaid coordinator the active UE's IDs and their crosstalk. Step 780 showsthat poorly separated UEs combinations may be addressed viacoordinator's initiated info to the Basebands.

FIG. 8 is a high level flowchart illustrating another aspect inaccordance with some embodiments of the present invention. The proceduremay include the following steps: Receiving a batch of UEs to bescheduled for DL MU-MIMO 805; Using estimated UEs mobility history, tag(i.e., assign with an identifier or indicator) UEs that appear static,or very slow moving 810; Defining minimum acceptableSignal-to-Interference-Ratio for MU-MIMO operation (M-SIR) 820;identifying conflicted UE as pairs assigned to the same resource blocksby different basebands, causing cross talk greater than M-SIR, to eitherone or both 830; List of Scheduled UEs is forward to Arbitrator bybasebands 840; Is one of them static (low mobility)? 850. In caseno—send rescheduling intervention request to base band 860 and in caseyes—ignore and send handover intervention request to baseband 870.

FIG. 9 is a high level flowchart illustrating yet another aspect inaccordance with some embodiments of the present invention. The proceduremay include the following steps: retrieve most recent SINR and QoS foreach scheduled UE 905; Calculate weights using ZF 910; comparingexpected resultant SINR to most recent SINR 915; checking whether allscheduled UEs have QoS requirements 920; in case yes—checking whetherthe expected SINR sufficient for each required QoS 925; in caseyes—proceed using the zero forcing (ZF) solution. In case not allscheduled UEs have QoS requirements 920—checking whether some of the UEshave QoS requirements is carried out 935 in case no—Perform ProportionalFairness MMSE weights calculation 945. In case yes—Perform ProportionalFairness MMSE that guarantees QoS to those UEs that have suchrequirement, while granting Proportional Fairness to the remaining UEs940.

After checking whether the expected SINR sufficient for each requiredQoS 925, if the answer is no, degrade one of the UEs DL service andrecalculated ZF for less number of UEs 950 and then checking whether theexpected SINR sufficient for each of the remaining UEs' QoS 955. In caseno, it will repeatedly degrade one of the UEs DL service andrecalculated ZF for less number of UEs 950 until the expected SINRsufficient for each of the remaining UEs' QoS. When it does, proceedingusing the ZF solution is carried out 930. However, in a case that theexpected SINR is sufficient for each of the remaining UEs' QoS,proceeding using the ZF solution is carried out 930.

For the case that the co-located basebands would not be modified tocommunicate with the said coordinator, the said front-end processor mayperform MU-MIMO operation by retrieving the active UE's ID and itsassigned resource blocks for a TDD system. The MU-MIMO performance maybe compromised. The simulation results in 610 and 620 show that thecompromise may be insignificant when the number of the antennas/radiosis larger than that of basebands (e.g., M>N).

In the above description, an embodiment is an example or implementationof the invention. The various appearances of “one embodiment”, “anembodiment” or “some embodiments” do not necessarily all refer to thesame embodiments.

Although various features of the invention may be described in thecontext of a single embodiment, the features may also be providedseparately or in any suitable combination. Conversely, although theinvention may be described herein in the context of separate embodimentsfor clarity, the invention may also be implemented in a singleembodiment.

Embodiments of the invention may include features from differentembodiments disclosed above, and embodiments may incorporate elementsfrom other embodiments disclosed above. The disclosure of elements ofthe invention in the context of a specific embodiment is not to be takenas limiting their used in the specific embodiment alone.

Furthermore, it is to be understood that the invention can be carriedout or practiced in various ways and that the invention can beimplemented in embodiments other than the ones outlined in thedescription above.

The invention is not limited to those diagrams or to the correspondingdescriptions. For example, flow need not move through each illustratedbox or state, or in exactly the same order as illustrated and described.

Meanings of technical and scientific terms used herein are to becommonly understood as by one of ordinary skill in the art to which theinvention belongs, unless otherwise defined.

While the invention has been described with respect to a limited numberof embodiments, these should not be construed as limitations on thescope of the invention, but rather as exemplifications of some of thepreferred embodiments. Other possible variations, modifications, andapplications are also within the scope of the invention. Accordingly,the scope of the invention should not be limited by what has thus farbeen described, but by the appended claims and their legal equivalents.

The invention claimed is:
 1. A system comprising: a number N co-locatedTime Division Duplex (TDD) base stations, each having a baseband modulecapable of a number K of multiple input multiple output (MIMO) ranks,wherein said N co-located TDD base stations are configured to share acommon antenna array and a common bank of radio modules, and whereinsaid N co-located TDD base stations are configured to operate over acommon frequency band; a front-end MIMO processor connected to said Nco-located TDD base stations and further coupled to said common antennasarray; and a back-end coordinator configured to optimize operation ofsaid N co-located TDD base stations, such that said N co-located TDDbase stations and said front-end MIMO processor implement a multi-usermultiple input multiple output (MU-MIMO) base station capable ofdynamically separating a coverage area into N*K spatial channels.
 2. Thesystem according to claim 1, wherein each of said N co-located TDD basestations has a scheduler and wherein said front-end MIMO processor isconfigured to carry out said dynamic separation based on said existingschedulers.
 3. The system according to claim 1, wherein said front-endprocessor is further configured to carry out on-site, periodicself-calibration on transmit and receive chains of each of said radiomodules or antennas, and to establish channel reciprocity based onresults of said self-calibration, so that uplink channel information ismapped into downlink channel estimation.
 4. The system according toclaim 3, wherein said front-end processor is further configured tocalculate a downlink pre-coding weights for at least some user equipment(UE), camped on the base station using said uplink channel information,said calibration data, and a MU-MIMO spatial multiplexing algorithm. 5.The system according to claim 1, wherein each of said N co-located TDDbase stations comprises an existing baseband module configured to carryout channel estimation of the channels served by the respective TDD basestations, and wherein said front-end processor is further configured tocarry out channel estimation for all the channels served by said Nco-located TDD base stations.
 6. The system according to claim 1,wherein said front-end processor is further configured to: deriveidentification for each of active user equipment (UE) and map saididentifications into respective channel estimations.
 7. The systemaccording to claim 6, where the said identification derivation is basedon decoding the UE ID in the front end processor and in the respectivebase station.
 8. The system according to claim 1, wherein each of said Nco-located TDD base stations has an existing baseband module configuredto detect uplink Physical Resource Block (PRB), each PRB being mappableto a distinct reference of frequency/time bins containing OFDM symbolsat a given TTI, wherein said front-end processor is further configuredto tag a channel estimation of a user equipment (UE) based on a binreceived from said existing baseband modules.
 9. The system according toclaim 1, wherein said front-end processor is further configured toderive uplink channel information relating to at least some userequipment (UE), wherein said uplink channel estimation is converted todownlink channel information by applying channel reciprocity, andfurther based on calibration data.
 10. The system according to claim 1,wherein a communication link established between each base station'sbaseband and the front end processor, over which schedulers plans aretransferred in advance, to the front end processor, and to thecoordinator, using UE ID or UE reference via bin at a given TTI, wheresaid scheduled UEs per TTI, per PRB, are processed by the MU-MIMOmechanism.
 11. The system according to claim 1, wherein a communicationlink between said base stations and the front end processor is used toinform it of a quality of service (QoS) and the signal to interferenceplus noise (SINR) of each of the scheduled UEs in both down and uplinks.
 12. The system according to claim 1, wherein the MU-MIMOprocessing takes into account: the quality of service (QoS) and thesignal to interference plus noise (SINR) levels of the UEs to bescheduled, and chooses whether to use Zero Forcing or minimal meansquare error (MMSE) algorithms for antennae weights calculation.
 13. Thesystem according to claim 1, wherein the MU-MIMO uses Zero Forcingwhenever signal to interference plus noise SINR levels of the userequipment (UEs) to be scheduled are approximately at generic SINR level,where generic means service that could have been provided solely to oneUE.
 14. The system according to claim 1, where the MU-MIMO processchecks quality of service (QoS) tagging of the scheduled UEs, and ifnone is set, an MMSE weight calculation is done based on a proportionalfairness approach.
 15. The system according to claim 1, wherein theMU-MIMO process checks quality of service QoS of the schedules UEs, andif all are tagged for the QoS, a minimal mean square error (MMSE)calculation is performed and prospective SINRs are compared to thoserequired by the various QoS, and if found sufficient, the weightscalculation is actually used, and if not, one of the UE's service isdegraded, so that the other QoS requirements are fulfilled, and if not asecond UEs service is degraded.
 16. The system according to claim 1,wherein the MU-MIMO process checks QoS of the schedules UEs, and if someof the UEs are tagged for QoS and some other are not, then the QoS onestake priority, and the non QoS tagged ones are served according to aproportional fairness approach.
 17. The system according to claim 1,wherein each of said N co-located TDD base stations has an existingbaseband module, and wherein said back-end coordinator is furtherconfigured to receive from said front-end-processor an informationregarding poorly separated user equipment (UEs) being UEs that havecross talk higher than a predefined level, and notifies at least one ofthe existing baseband modules to modify a schedule plan.
 18. The systemaccording to claim 1, wherein each of said radio modules having a localoscillator, wherein said front-end processor is further configured tophase lock on all said local oscillators, in order to achieve phasecoherency, sufficient for beamforming.
 19. The system according to claim1, wherein said front-end processor is further configured to send userequipment (UE) uplink channel information and UE identification to saidback-end coordinator for checking and resolving conflicts occurringwhenever a crosstalk level exceeds predefined levels by sending anintervening request to the basebands.
 20. The system according to claim19, wherein said intervention is achieved by requesting some schedulersto proceed as planned, while others are requested to refrain fromscheduling a given UE for certain PRBs, over a given up and comingTransmission Time Interval (TTI).
 21. The system according to claim 1,wherein each of said N co-located TDD base stations has an existingscheduler and wherein said back-end coordinator is further configured toidentify poorly separated UEs the are either static or have lowmobility, and handover one or more of them to other base station in thesystem, and repeat this procedures for other poorly separated UEs, sothe occurrence of poorly separated UEs is minimized.
 22. The systemaccording to claim 1, wherein the antenna array comprises M antennas,serving N baseband units each supporting rank-K MIMO, where M≧N*K, i.e.the baseband population is sparse, thus increasing the MU-MIMO degreesof freedom and promoting higher and better spatial separation.
 23. Thesystem according to claim 22, such handover events are limited to UEswhose history indicated static position, where history is measure intens of seconds, and where static is defined as channel estimationvariation that has approximately zero mean.